The present invention relates to timing discriminators; and, more particularly, it relates to a timing discriminator that uses leading-edge extrapolation for the recovering of timing information from slow-rising pulses.
To obtain good timing information from slow-rising pulses is an old problem in pulse spectroscopy, and especially in more recent high-energy calorimeter applications. A slow-rising pulse is considered to be a pulse that rises to its peak value in a time of 50 nanoseconds or greater. The output pulse in this type of detector may have a long risetime for several reasons, such as the following:
1. The pulse is derived from many proportional chamber wires ganged together, with drift times varying for any given particle trajectory. PA1 2. The preamplifier is often designed to integrate the incoming charge. PA1 3. Additional high frequency filtering is sometimes used to stabilize the preamplifier in order to reduce high-frequency noise or to provide a pulse shape more suitable for sampling.
Furthermore, these factors may cause a variation in the overall shape of pulses.
To overcome the above problems, the prior art takes advantage of the fact that any pulse carries information about pulse timing which is more accurate than its risetime. This information is recovered by means of a timing discriminator, while imposing an amplitude requirement, or threshold to the input pulse. In general, a timing discriminator is a circuit which produces an output pulse in a fixed relation to the time of the input pulse, if the input pulse is greater than a threshold voltage. Conventional techniques, like zero-crossing and constant fraction, are actually used in connection with timing discriminators to recover pulse risetime information. Discriminator circuits are known in the art, such as the one disclosed in U.S. Pat. No. 3,676,783, issued July 11, 1972, to Setsuro Kinbara et al. In this patent, the pulse-shape discriminating circuit described is suitable for use in the measurement of pulses from radiation detectors. This circuit comprises an input terminal for receiving an input signal; a plurality of characteristic circuits having some characteristics such as differentiation, delay, attenuation, integration, etc.; a plurality of amplitude-discriminating circuits to compare the amplitude of pulses from the characteristic circuits; and a mixing circuit to combine step pulses from the amplitude-discriminating circuits. This device is used to measure an input pulse risetime, independently of amplitude variations, by discriminating between the shape of the pulses. However, this device results in extensive, complex and expensive circuitry.
In the publication, "A Time Compensation Method for Coincidences Using Large Coaxial Ge(Li) Detectors," by Fouan and Passerieux, Nuclear Instruments and Methods 62 (1963) 327-329, a method is described to compensate for the time jitter due to slow and variable pulse rise times. However, no practical embodiment is described here and the complex exposition of the technique affords no details for the development of an embodiment of a timing discriminator circuit.
It is, accordingly, a general object of the invention to improve the recovering of timing information from slow-rising pulses by using leading edge extrapolation in real time mode.
Another object is to improve actual discriminators for high-energy calorimeter applications where long risetime and nonlinear rise are usually expected.
Another object is to provide a practical embodiment with compact, simple and inexpensive circuitry for timing discriminator purposes using the leading edge technique.